1. Field of the Invention
This invention relates to a semiconductor device with a chalcogenide layer containing zinc or cadmium.
2. Description of the Related Art
A chalcogenide semiconductor containing zinc or cadmium has a band gap corresponding to the visible light region, and hence is expected to be used as a visible light-emitting semiconductor device. In particular, in a material mainly consisting of a zinc compound, light emission of wavelengths ranging up to a blue region has been confirmed, and this material is now being developed as a material of a shorter-wavelength light-emitting device.
However, incomplete ohmic contact between a chalcogenide layer and an electrode metal arises as a serious problem in the chalcogenide semiconductor device. Specifically, good ohmic contact cannot be made between the chalcogenide layer and Au or Pt, which are now widely used as electrode metals. Thus, a high contact resistance may occur between the electrode metal and chalcogenide layer, necessitating an increase in the voltage applied therebetween. As a result, a great amount of heat may be generated, causing defects in the crystal and hence deteriorating the semiconductor device.
To solve the above problem, the following light-emitting device has been proposed in Published Unexamined Japanese Patent Application No. 1-187885. As is shown in FIG. 15 in the application, an n-type ZnSSe crystal layer 152, a p-type ZnSSe crystal layer 153, and a p-type GaAs crystal layer 154 are formed on an n-type GaAs substrate 151 in the order mentioned. Reference numerals 155 and 156 denote electrodes. In this light-emitting device, the contact resistance can be reduced by growing a III-V group semiconductor layer on a p-type II-VI group semiconductor layer, and forming an ohmic electrode on the III-V group semiconductor layer.
This device, however, has the following problems:
1) The degree of valence band discontinuity is high between the III-V group compound and the II-VI group compound, which makes it difficult for holes to flow, and hence reduces the light-emission efficiency; PA1 2) Since the GaAs crystal layer 154 is formed on the p-type ZnSSe crystal layer 153 as a p-type II-VI group semiconductor layer, Ga atoms in the GaAs layer 154 will diffuse into the layer 153, with the result that the conductivity type of the layer 153 will be shifted from the p type to the n type; and PA1 3) Since the growth temperature of a III-V group compound such as GaAs is higher than that of a II-VI group compound, mutual diffusion will be caused by heat at the pn junction between the II-VI group semiconductor layers while the III-V group semiconductor layer is being grown. This diffusion may adversely affect the pn junction. PA1 a p-type first layer consisting of a chalcogenide semiconductor containing zinc or cadmium; PA1 a semiconductor supporting layer consisting of an n-type semiconductor; PA1 a second layer formed on the first layer and between the first layer and the first electrode, the second layer being made of a mixed crystal semiconductor consisting of group II, III, V, and VI elements or being consisting of a II-V group compound semiconductor.